Various industrial applications of a near-infrared ray absorbing coloring matter having absorption in a near-infrared region of 700 to 2000 nm have been conventionally under consideration. Specifically, it is used for application to light information recording medium such as CD-R, for printing application such as thermal CTP, fixing of a flash toner and laser thermal recording, and for heat blocking film and the like. Additionally, it is also used for near-infrared ray cutting filters used for PDP filters and the like, films for plant growth regulation, and the like, by taking advantage of the properties that it selectively absorbs lights in a certain wavelength range. On the other hand, it is also possible to use a near-infrared ray absorbing coloring matter as a near-infrared ray absorbing ink by dissolving or dispersing it in a solvent. A printed matter with said ink is difficult to recognize by visual observation and can be read only by a near-infrared ray detector or the like, so said ink is used for printing or the like for the purpose of, for example, anti-counterfeiting or the like.
Inks are classified broadly into water-based inks and organic solvent-based inks on the basis of the main component in a solvent, and use of a water-based ink is strongly desired in terms of environmental problems so development of a near-infrared ray absorbing coloring matter which is soluble in water is desired.
As a near-infrared ray absorbing coloring matter, immonium coloring matter, diimmonium coloring matter, dithiol metal complex coloring matter, cyanine coloring matter and the like are known, and particularly, as a near-infrared ray absorbing coloring matter having a high fastness, naphthalocyanine coloring matter is known.
A known naphthalocyanine coloring matter is a pigment the most of which is insoluble in a solvent or a coloring matter exhibiting solubility only in an organic solvent (see Patent Literature 1, for example), and almost no naphthalocyanine coloring matter exhibiting good solubility in water has been known.
In the examples of Patent Literatures 2 and 3, a water-soluble naphthalocyanine coloring matter is obtained by sulfonation of a naphthalocyanine coloring matter. This compound is dissolved in water, but when it is formulated into a water-based ink composition, its storage stability is poor and it is not in practical use.
Patent Literature 1: JP 2507786 B
Patent Literature 2: JP H2-167791 A
Patent Literature 3: U.S. Pat. No. 7,122,076